As-for-Sb and Sb-for-As anion exchange reactions have been investigated by the exposure of GaSb surfaces to As 2 and As 4 species and by the exposure of GaAs to Sb 2, respectively. The effect of surface temperature, anion soak time, and anion species (either As 2 or As 4) on the chemistry governing the anion exchange reactions during GaAs ySb 1-y/GaSb and GaSb yAs 1-y/GaAs heterostructure formation by molecular beam epitaxy is examined. It is found that when GaSb surfaces are exposed to arsenic, the anion exchange reaction competes with the formation of isoelectronic compounds, AsSb y, which form clusters precipitating in the GaAs ySb 1-y/GaSb heterostructures. The relative amount of GaAs and AsSb y depends on the surface temperature, the As soak time, and on the As species, i.e., As 2 or As 4. We observe specific process conditions that minimize AsSb y formation, yielding more abrupt heterojunction interfaces. In the case of the Sb 2/GaAs system, the Sb-for-As anion exchange does not occur to a significant degree, but surface segregation of antimony is found. A chemical model for the As-for-Sb anion exchange reaction is proposed. © 2006 American Institute of Physics.